In practice, in the event of a frontal impact at a speed less than four kilometers/hour, the bumper and the crash-boxes are deformed in the elastic range and return to the initial position thereof without damage. In the event of an impact at a speed greater than four kilometers/hour, the bumper and the crash-boxes are subject to plastic deformation which makes it possible to at least partially absorb the energy from the impact. When the impact occurs at a speed remaining below approximately fifteen kilometers/hour, the energy absorbed makes it possible to protect the rest of the structure of the vehicle (reparability threshold). Beyond this, in the event of impacts at a speed above fifteen kilometers/hour, the structure of the shaft is damaged and the resulting repair costs are high.
Through numerous prior art documents, such as U.S. Pat. No. 6,003,930, US2004/0201254, U.S. Pat. No. 6,258,465, EP 1 041 165, EP 1 688 312, US2003/0207143, US2001/0037844, crash-boxes are known which have an elongated profiled portion in the longitudinal direction which is deformed by successive folding under the effect of the compression force generated by the impact. This deformation mode, referred to as “progressive folding”, is the result of a set of micro-instabilities giving rise to local bending of the longitudinal walls of the profiled portion, which are deformed plastically, causing regularly distributed lobes to appear. Advantageously, the profiled portion is an extruded profile member, typically made of aluminum alloy, the cross-section whereof is designed such that progressive folding can occur along the entire effective length of said profiled portion and absorbs a large proportion of the kinetic energy. The shape and thickness of the profile cross-section, the type of extruded alloy are chosen to obtain regular progressive folding preventing a major instability, such as buckling, from resulting in the sudden formation of a swivel joint (swiveling) whereby the profile is rapidly folded and crushed before being able to absorb all the intended amount of kinetic energy.
“Crash test” protocols have been defined by motor vehicle manufacturers and insurance companies to improve the safety conditions applied to vehicles. Initially, these tests simulated frontal impacts, i.e. impacts giving rise to perfectly longitudinal forces. Recently, these “crash tests”, notably Euro-NCAP or RCAR, follow a protocol (see for example “RCAR Low-speed structural crash test protocol”—Issue 2.2 Jul. 2011) wherein the impact is no longer perfectly frontal but offset: as illustrated in FIG. 1, the vehicle (1) undergoing the test is projected against a wall (2) placed on the driver's side such that it offers an obstacle over more than 40% of the width of the vehicle (U>0.4*B) and forms an angle (α) of 10° with respect to the perpendicular (P) to the longitudinal axis (L) of the vehicle.
The new EuroNCAP and RCAR impact test protocols apply impacts which are not merely frontal but involve the presence of transverse forces and torques acting on the absorber. The new conditions of these tests increase the risk of elastic instability such as buckling of the absorber. When the buckling results in pivoting of the absorber in a horizontal plane, the term “skewing” is used. When the buckling results in pivoting of the absorber in a vertical plane, the term “verticalization” is used.
In general, as illustrated in numerous documents, such as GB2304651, EP0734908, WO97/03865, EP0894575, US2002/0113447, U.S. Pat. No. 6,003,930 and EP1688312, the end of the absorber is provided with a transverse plate, which is attached to a transverse plate rigidly connected to the structure (front or rear) of the vehicle. The European patent application EP 1 717 107 differs from the prior art by describing an absorber which is attached to a side rail after having been partially inserted into the recess of said side rail, which is presented in the form of a hollow profile member. The absorber has a first end whereon the bumper crossbeam is attached and a second end which is inserted into the recess of the side rail. Once inserted along a length E0 substantially equal to a characteristic dimension of the cross-section of the absorber, for example the diameter of the circumscribed circle of said cross-section, this second end is attached to the side rail with attachment means which secure the longitudinal walls facing the crash-box and the side rail. These attachment means are typically bolts through said facing longitudinal walls of the crash-box and the side rail.
Inserting the end of the absorber inside the side rail, as proposed in EP 1 717 107, has the advantage over prior solutions, which consisted of attaching the absorber to the side rail using a transverse plate, of reducing the risk of buckling of the absorber in the event of impact. However, tests have progressed and have become more severe, by introducing lateral and/or dissymmetric forces during the impact. These forces significantly increase the risk of skewing and verticalization of the crash-box in the event of an impact.
The aim of the present invention is that of finding conditions for attaching such an absorber which make it possible to substantially reduce the risks of skewing and verticalization, in particular in the event of impacts involving lateral forces.
The invention firstly relates to an impact-absorbing structure for a vehicle, the structure of which comprises at least one side rail intended to support a bumper crossbeam, said side rail being a hollow profile member arranged according to the longitudinal axis of said vehicle and having at least two opposing longitudinal walls, said impact-absorbing structure comprising:
a. said bumper crossbeam;
b. at least one absorber placed between said side rail and said bumper crossbeam, said absorber being a hollow profile member intended to be deformed in the event of an impact, having at least two opposing longitudinal walls, one end of said profile member being intended to be inserted into the open end of said side rail such that said opposing longitudinal walls of said absorber are arranged facing and in the vicinity of said opposite longitudinal walls of the side rail, said end of said profile member being attached to said side rail using attachment means, typically assemblies of bolts+nuts, arranged transversely, perpendicular to said opposing longitudinal walls of the absorber and the side rail,
c. an attachment insert provided with opposing surfaces which, when said attachment insert is inserted into said absorber, are arranged facing and in the vicinity of said opposing longitudinal walls of the absorber,
characterized in that each of said opposing surfaces of the insert is provided with two transverse recesses arranged perpendicular to said opposing surfaces, intended to engage with said attachment means, and offset relative to one another both in the longitudinal direction and in the transverse direction perpendicular to the direction of the axis of said transverse recesses.
In general, the front structure of a motor vehicle comprises a shaft including two side rails supporting a bumper crossbeam together. The impact-absorbing structure or CMS results from the assembly of said bumper crossbeam and two absorbers serving as intermediate elements between the bumper crossbeam and the side rails. The impact-absorbing structure is positioned on the front (or rear) structure of the vehicle such that each absorber is inserted between a side rail and said bumper crossbeam. The absorber is thus both an element whereby the bumper crossbeam is attached to the front structure of the vehicle and a deformable element designed to absorb a certain quantity of kinetic energy in the event of impact.
The absorber used within the scope of the invention is a hollow profile member similar to that described in EP 1 717 107, having opposing longitudinal walls which, when it is inserted into the side rail, are situated facing and in the vicinity of opposing longitudinal walls of said side rail. When it is attached to the front (or rear) structure of the vehicle, it is arranged along the longitudinal axis of said side rail, one of the ends thereof being inserted into the open end of said side rail and attached thereto using attachment means, typically screws or bolts provided with nuts which are arranged transversely, perpendicular to said facing longitudinal walls of the absorber and the side rail. Said facing longitudinal walls of the absorber and the side rail are placed in contact and secured by screwing or by fastening nuts on bolts through both ends of the side rail and the absorber.
According to the invention, the impact-absorbing structure also comprises an attachment insert intended to be inserted into the absorber with a view to engaging with said attachment means. For this purpose, the insert comprises two opposing surfaces, which, when the insert is inserted into the absorber, are situated facing and in the vicinity of opposing longitudinal walls of said absorber. These opposing surfaces are intended to serve as internal bearing members against which the opposing longitudinal walls of the absorber abut, the latter being driven, with the opposing longitudinal walls of the side rail, towards said bearing members when nuts are fastened onto said bolts.
According to the invention, each of the opposing surfaces of the insert is provided with two transverse recesses arranged perpendicular to said opposing surfaces, intended to engage with said attachment means, and offset relative to one another both in the longitudinal direction and in the transverse direction perpendicular to the direction of the axis of said transverse recesses. These transverse recesses are either recesses provided with a screwing thread, or bores intended for the insertion of bolts through either side of the side rail, the absorber and the insert. Preferably, the transverse recesses of one of the opposing surfaces are aligned with the transverse recesses of the other opposing surface. In some embodiments of the invention, the aligned transverse recesses join to form a bore through the insert from one or the other of the opposing surfaces.
The number and arrangement of the transverse recesses on each of the opposing surfaces of the insert, which determines the spatial arrangement of the attachment means, is an important parameter influencing the buckling strength of the absorber, in particular in the event of offset impacts. If, as in some embodiments of EP 1 717 107, a plurality of inserts are used, each of these inserts being provided with a single transverse recess, it is observed that, despite the support role played by the inserts, the risk of verticalization or skewing remains high. This is probably linked, at least in part, to the fact that each insert has independent behavior.
When using, not a plurality of inserts as in EP 1 717 107, but a single insert provided with said transverse recesses, superior results are obtained, but these are only significantly superior if the plane defined by the axes of the transverse recesses of each surface is neither parallel nor perpendicular to the longitudinal direction. With reference to FIG. 3a, where E denotes the component along the longitudinal direction (L) of the offset between the two transverse recesses and V denotes the component of the same offset along the transverse direction (Z) perpendicular to the direction (Y) of the axis of the transverse recesses, E and V must be strictly greater than 0. Preferably, these components are the greatest possible, compatible with the geometric constraints applied.
Advantageously, the penetration length E0 of the end of the absorber in the side rail is at least equal to a characteristic dimension of the cross-section of the absorber, for example the radius of the circumscribed circle of said cross-section or half the greatest dimension of said cross-section (half of the length thereof if it is substantially rectangular). The component E along the longitudinal direction of the offset between the two transverse recesses of the insert is preferably greater than E0/4, more preferably greater than E0/3.
Preferably, said opposing longitudinal walls of the absorber are associated with the greatest dimension V0 of the cross-section of said absorber to maximize the contact surface area between the opposing walls of the side rail and the opposing walls of the absorber. Under these conditions, the transverse direction perpendicular to the axis of the transverse recesses is the direction of the greatest dimension of the cross-section. The component V along the transverse direction perpendicular to the axis of the transverse recesses of the offset between the two transverse recesses of the insert is preferably greater than V0/4, more preferably greater than V0/2. As stated above, E0 is preferably greater than V0/2, such that the component E along the longitudinal direction of the offset between the two transverse recesses of the insert is preferably greater than V0/8, more preferably greater than V0/6.
In the preferred embodiments of the invention, the greatest dimension V0 of the cross-section of the absorber is arranged vertically and said opposing longitudinal walls of the absorber are vertical walls extending along a height substantially equal to the overall height (H0) of the absorber. In this case, the direction of the axis of the transverse recesses is horizontal and perpendicular to the longitudinal direction. The penetration E0 being preferably greater than H0/2, the component along the longitudinal direction of the offset between the two transverse recesses of the insert is preferably greater than H0/8, more preferably greater than H0/6, or H0/4. Advantageously, the component along the vertical transverse direction (which is perpendicular to the horizontal direction of the axis of the transverse recesses) of the offset between the two transverse recesses of the insert is greater than H0/4, more preferably H0/2.
In the embodiments described hereinafter, the transverse recesses of one of the opposing surfaces are aligned with the transverse recesses of the other opposing surface. These are either recesses provided with a screwing thread intended to engage with screws, or bores intended for the insertion of bolts through either side of the side rail, the absorber and the insert.
In a first embodiment, the insert comprises solid areas surrounding said transverse recesses, extending from one of said opposing surfaces to the other and which are interconnected by a thin wall. The absorber is a hollow profile member which comprises three chambers separated from one another by two horizontal internal partitions. The thin wall of the insert has an inverted U shape wherein the branches are not equal, which comprises two horizontal wall portions interconnected by a vertical wall portion. Following the insertion of the insert into the absorber, the horizontal wall portions are arranged facing said horizontal internal partitions such that the thin U-shaped wall overlaps with the two horizontal internal partitions of the absorber and the insert is inserted until the vertical portion comes to a stop against said partitions. The absorber is attached to the side rail either by screwing (the transverse recesses are in this case recesses provided with a screwing thread) or by bolting (the transverse recesses are in this case through bores for inserting the bolts through the side rail-absorber-insert assembly). As such, a stable assembly is obtained, whereby the attachment of the absorber onto the side rail provides conditions similar to those of flush fitting, making it possible to reduce the risks of skewing and verticalization of said absorber in the event of a deflected impact.
In a second embodiment, the insert comprises two opposing solid areas, each solid area comprising one of said opposing surfaces. The transverse recesses are either recesses provided with a screwing thread, or bores aligned each passing through one of said solid areas. The absorber is attached to the side rail either by screwing or by bolting. A stable assembly is also obtained making it possible to substantially reduce the risks of skewing and verticalization of said absorber in the event of a deflected impact.